This invention relates to a thermo-transfer sheet, a label utilizing the sheet and being provided with a printed picture superior in friction-proof property, oil-proof property, water-proof property, and heat-proof property, and a method for manufacturing the same.
Recently, various kinds of commercial products have applied with labels on which various kinds of letters, numerals, symbols, patterns, and optically readable images, particularly in the form of bar codes, for the selling and managing of the products. Labels or the like characters have been widely utilized for the manufacturing, quality control, storing, and delivery of the products.
In one example of the conventional bar code, a number of lines constituting a bar code are recorded on a bar code label sheet and such bar code label sheets have been prepared in mass production by printing the bar codes with usual printing ink. On the other hand, in a small product production, a thermo-transfer printer has been utilized as a convenient printing method. In the bar code printing method by the utilization of such thermo-transfer printer, a thermo-transfer sheet consists of a base film having one surface on which a thermally fusible ink layer is formed.
The basic film of such a conventional thermo-transfer sheet is prepared by a paper made of paraffin paper or condenser paper having a thickness of 10 to 20 .mu.m, or a plastic film such a polyester or cellophane having a thickness of 3 to 20 .mu.m and by coating a thermally fusible ink layer prepared by mixing wax with coloring agent such as pigment or die.
Such a known thermo-transfer sheet is formed by heating the sheet from the rear side in accordance with an image by a thermal head and fusing the transfer ink layer on an image receiving sheet. The ink layer is formed of a material having a low melting point for the reason that the image is formed on the material to be transferred due to the adhesive property, caused by the heating, of the ink layer. For this reason, the image on the thermally transferred material is inferior in the friction-proof property, the solvent-proof property, and the heat-proof property and, accordingly, the image is easily worn or peeled off by friction, or white color portions near the printed portions of the image are also easily damaged or contaminated.
Such adverse problems are significant for the thermo-transfer sheets utilized in art fields in which the superior friction-proof property, solvent-proof property and heat-proof property are required, for example, in the preparation of the bar codes.
For instance, the conventional thermo-transfer type bar codes will not be applied to portions at which products often contact to each other, mechanical oil exists, or the bar codes are often liable to be heated.
In order to solve these problems, there is provided a method in which a transparent cover film made of a thin polyester film, for example, is laminated to cover the surface of the bar code label to protect the same. According to this method, it is possible to manufacture a bar code label superior in the friction-proof property, but there remains a problem such that the printed wax ink layer is again softened or fused during the fusing, heating or pressing process of the bonding agent at a time when the cover film is laminated. These adverse phenomena may finally result in the diffusing of the printed image and the lowering of the resolving ability thereof.
In order to solve the problem described above, there is provided a method in which a thermo-plastic resin having compatibility with an ink vehicle of a thermo-transfer ink sheet is applied to a material to be transferred, such as disclosed in the Japanese Patent Laid-open Publication Nos. 63-193884 and 63-194981. However, with this method, the problems of the diffusing of the printed image and the lowering of the resolving ability cannot be satisfactorily solved. Moreover, in this method, the transparent cover film laminating process is additionally required, which results in the increasing of the manufacturing cost of the bar code label.
The described problems may be common to the labels on which printed images or pictures such as letters, numerals, symbols, patterns, or optically readable images other than the bar code labels are present.